1. Field of the Invention
Aspects of the present invention relate to a power supply and an image forming device having the same, and, more particularly, to a power supply, which supplies a voltage having a plurality of potential levels in order to improve cross regulation between outputted voltages and at the same time reduce electric power consumption, and an image forming device having the same.
2. Description of the Related Art
Generally, power supplies using switching methods (hereinafter referred to as “switching power supplies”) have been widely utilized. In the switching methods, a direct current obtained by rectifying and smoothing a commercial alternating current is switched by a predetermined high frequency (for example, about 100 kHz) to be converted to a desired voltage by a high efficiency transformer. A method for controlling output voltages of the switching power supply uses a pulse width modulation (PWM) control method to control the duty cycle of a switching pulse according to the change of output voltage, a frequency control method to control the frequency of the switching pulse, and phase control method to control a phase of the switching pulse.
FIG. 9 is a view to illustrate an example of the switching power supply using the PWM control method. The switching power supply 10 in FIG. 9 using the PWM control method has a switching circuit 12 including one or more switches formed on a primary winding of a transformer 11. By turning the switching circuit 12 on or off, the switching power supply 10 converts a direct current input voltage DC_IN, which is applied to the primary winding of the transformer 11 and is not rectified, to a direct current output voltage DC_OUT. The generated direct current output voltage DC_OUT is rectified by a diode D1 and a capacitor C1 inside an output part 13 connected to a secondary winding of the transformer 11 (Vout) to be outputted. In the device shown, the switching circuit 12 is turned on or off in accordance with a control signal of an output controller 14 to modulate the pulse width of the switching pulse in response to the output signal from the output part 13.
In the switching power supply in FIG. 9, since the rectified end connected to the secondary winding has a simple structure, a small number of parts are used. Therefore, it is suitable to apply the switching power supply to a multi-output power supply that outputs voltages with different potential levels. FIGS. 10 and 11 are views to illustrate examples of conventional multi-output power supplies. In FIG. 10, a conventional multi-output power supply 20 according to the first example has power supplies 30 and 40. Each of the power supplies 30 and 40 has the same constitution as that of the power supply illustrated in FIG. 9. For example, each of the power supplies 30 and 40 has first and second input parts 31 and 41 receiving the direct current input voltage DC_IN; first and second power converters 32 and 42 each have the transformer; first and second output parts 33 and 43 rectifying the direct current output voltages DC_OUT1 and DC_OUT2 outputted from the power converters 32 and 42, respectively, to output the direct current output voltages; and first and second output controllers 34 and 44 modulating the pulse width in response to the direct current output voltages Vout1 and Vout2 outputted from the output parts 33 and 43, respectively, to output the pulse width. When the power supplies 30 and 40 are used in an external apparatus (for example the image forming device 50 shown, such as a printer,) an On/Off control signal to interrupt operation of the second power supply 40 is provided to the second output controller 44 to prevent an output of the switching pulse from the second output controller 44 in a standby mode when a print engine part 52 does not operate.
When the multi-output power supply 20 has the above-described structure, the multi-output power supply 20 has multiple input/output parts 31, 33, 41 and 43, the power converters 32 and 42, and the output controllers 34 and 44 to multi-output the voltage. Accordingly, a large number of parts are used, increasing the size of the power supply 20 and resulting in an increase in manufacturing cost.
Referring to FIG. 11, a conventional multi-output power supply 70 according to the second example has an input part 71 receiving the direct current input voltage DC_IN; a power converter 72 having a transformer having one primary winding and two secondary windings; first and second output parts 73 and 74 rectifying the direct current output voltages DC_OUT1 and DC_OUT2 outputted from the power converter 72, respectively, to output the direct current output voltages Vout1 and Vout2; and an output controller 75 modulating the pulse width of the switching pulse in response to the first direct current output voltage Vout1 of the second output part 73 to output the pulse width. When the multi-output power supply 70 has the above-described structure, the constitution of the input part 71 is simpler than the multi-output power supply 20 illustrated in FIG. 10, and is further simplified by including one power converter 72 and one output controller 75. Therefore, the manufacturing costs of the power supply 70 can be reduced.
However, in the multi-output power supply 70 according to the second example, the pulse width of the switching pulse inputted to the input part 71 for controlling the direct current output voltages Vout1 and Vout2 is modulated depending on the direct current output voltage Vout1 outputted from the first output part 73, so that the voltage stability of the direct current output voltage Vout2 of the second output part 74 decreases.
In particular, if the pulse width of the switching pulse is varied in order to compensate for the impedance when the impedance of the load of the first output part 73 or the second output part 74 changes, the voltage stability of the direct current output voltage at the other side (that is, the cross regulation) is reduced because one primary winding is used together with the secondary winding.